An online LiFePO4 battery impedance estimation method for grid-tied residential energy storage systems

There has recently been a significant interest directed towards residential battery storage systems mainly motivated by high penetration of renewables, the low cost and high efficiency of power electronic devices, and the advancements in the safety and energy density of the batteries, especially Lithium-Ion (Li-Ion) batteries. Furthermore, the possibility for the end user to become a utility-independent entity with the capacity to overcome power outages and tariff rises is even further propelling this fast growing industry. Lithium iron phosphate (LiFePO₄) battery is one of those technologies chosen to take the lead in residential battery storage due to its intrinsic safe performance, good energy density and price. This paper describes an online method for estimating the impedance of LiFePO₄ batteries when they are used in residential single phase energy storage systems. Single phase power systems have the intrinsic characteristics of delivering power at twice the frequency of the grid; by energy conservation principle, this pulsating characteristics is transferred directly to the current in the DC stage of the battery storage system. The proposed method takes advantage of this phenomenon and, without interrupting the energy conversion process or adding any external perturbation to the system, is able to characterize, in situ, the AC impedance behavior of the battery. Experimental results are provided to validate the proposed method and simulations show the potential applicability of this method in the assessment of the actual battery aging state.